1. Field of the Invention
The present invention relates to a photo-curable resin composition and a method for forming a concave-convex pattern. More particularly, the present invention relates to a photo-curable resin composition containing as a film-forming component a urethane-modified acrylic resin which can form a film simultaneously having heat resistance, softness and other properties, and a method for forming a concave-convex pattern, such as a diffraction grating or a relief hologram.
2. Background Art
Diffraction grating, relief holograms or the like have hitherto been produced by coating a photo-curable resin composition (coating composition) onto a substrate, for example, a polyester film, to form a photo-curable resin layer, imparting various concave-convex patterns to the photo-curable resin layer, exposing the resin layer to ultraviolet light or an electron beam to cure the resin layer, and then laminating a metal deposit or a layer having a refractive index different from the resin layer onto the surface of the formed concave-convex pattern to prepare a diffraction grating, a relief hologram or the like.
For example, in the formation of a relief hologram, an example of a method for imparting the concave-convex pattern comprises the steps of: providing a press stamper prepared from a master hologram with a desired concave-convex pattern formed thereon (hereinafter referred to simply as xe2x80x9cpress stamperxe2x80x9d); putting the press stamper on the photo-curable resin layer and pressing (embossing) the resin layer to transfer the concave-convex pattern of the press stamper onto the resin layer; and, in this state, exposing the resin layer to cure the resin layer, thereby fixing the concave-convex pattern. When a large number of relief holograms are formed in this way, a large number of embosses are formed by means of a single press stamper.
The conventional photo-curable coating composition is in many cases liquid and thus has poor storage stability, leading to a fear of the quality of the resultant concave-convex pattern, such as holograms, being unstable. Further, since the conventional photo-curable coating composition contains many additives, repetition of embossing a large number of times causes components of the coating composition to be deposited onto the press stamper. This spoils the shape of a fine concave-convex pattern of the press stamper, here again resulting in unstable quality of holograms or the like.
Diffraction gratings and relief holograms have hitherto been used for decoration of or prevention of forgery of various cards, securities and other purposes. With the expansion of applications thereof, excellent strength, heat resistance, scratch resistance, water resistance, chemical resistance, and adhesion to substrates, as well as conformability to flexing or expansion/contraction of objects with diffraction gratings or relief holograms being applied thereon, have become required of the diffraction gratings and the relief holograms. In particular, in the case of diffraction gratings or relief holograms comprising the conventional photo-curable coating composition, the resin used loses its softness upon photo-curing, and, consequently, flexing or expansion/contraction of an object with the diffraction grating or relief hologram being applied thereon leads to the loss of functions as the diffraction grating or relief hologram.
In order to solve these problems, the present inventors have succeeded in providing, using material systems disclosed in Japanese Patent Laid-Open No. 156273/1986 and Japanese Patent Publication No. 54502/1993, photo-curable resin compositions which can form films having excellent strength, heat resistance, scratch resistance, water resistance, chemical resistance, and adhesion to substrates and, at the same time, can form diffraction gratings, relief holograms or the like having conformability to flexing or expansion/contraction of an object with the diffraction grating or relief hologram being applied thereon.
The reactive resin described in Japanese Patent Laid-Open No. 156273/1986 is prepared by addition bonding hydroxyethyl methacrylate again to a resin having hydroxyethyl methacrylate units in its main chain through a diisocyanate to introduce a double bond into the resin. Therefore, it is difficult to introduce the double bond as designed. Further, the reactive resin has an additional problem that the resin is likely to gel due to influence of a very small amount of water in the system. On the other hand, the reactive resin described in Japanese Patent Publication No. 54502/1993 is slightly inferior in water resistance due to the melamine skeleton, and, when a methylol group is present, unfavorably undergoes hydrolysis to evolve formaldehyde.
Accordingly, it is an object of the present invention to provide a photo-curable resin composition which can form films having excellent strength, heat resistance, scratch resistance, water resistance, chemical resistance, and adhesion to substrates and can form diffraction gratings, relief holograms or the like having conformability to flexing or expansion/contraction of an object with the diffraction grating or relief hologram being applied thereon.
The above object can be attained by the following present invention. Specifically, according to the present invention, there is provided a photo-curable resin composition comprising a urethane-modified acrylic resin represented by structural formula (I) or (II) and a release agent as indispensable components: 
wherein five R1""s each independently represent a hydrogen atom or a methyl group, R2 represents a C1-C16 hydrocarbon group, X and Y each represent a straight-chain or branched alkylene group, and, with the sum of l, m, n, and o being 100, l is an integer of 20 to 90, m is an integer of 0 to 50, n is an integer of 10 to 80, and o is an integer of 0 to 20; and 
wherein Z represents a group having a bulky cyclic structure, six R1""s each independently represent a hydrogen atom or a methyl group, R2 represents a C1-C16 hydrocarbon group, X and Y each represent a straight-chain or branched alkylene group, and, with the sum of l, m, n, o, and p being 100, l is an integer of 20 to 90, m is an integer of 5 to 80, n is an integer of 0 to 50, o+p is an integer of 10 to 80, and p is an integer of 0 to 40.
According to a preferred embodiment of the present invention, the photo-curable resin composition further contains an organometal coupling agent.
The present invention further provides a method for forming a concave-convex pattern, comprising the steps of: coating the above photo-curable resin composition onto at least one side of a substrate and drying the coating to form a photo-curable resin layer; embossing the surface of the photo-curable resin layer; and then exposing the resin layer to cure the resin layer.
According to the present invention, use of a specific urethane-modified acrylic resin as a main film-forming component in the photo-curable resin composition can provide a photo-curable resin composition which can form films having excellent strength, heat resistance, scratch resistance, water resistance, chemical resistance, and adhesion to substrates and particularly excellent capability of a metal being vapor deposited thereon, and can form diffraction gratings, relief holograms or the like having conformability to flexing or expansion/contraction of an object with the diffraction grating or relief hologram being applied thereon.